1. Field of the Invention
The present invention relates to a method for producing a nitride crystal and a nitride crystal. The present invention also relates to a method for producing a nitride crystal to be grown by using a nitride crystal starting material having an oxygen concentration within a specific range and a nitride crystal produced by the method.
2. Description of the Related Art
As a method for producing a nitride crystal, an ammonothermal process is known. An ammonothermal method is a method for producing a desired material using a nitrogen-containing solvent such as ammonia or the like in a supercritical state and/or a subcritical state and utilizing the dissolution-precipitation reaction of the starting material therein. In the method, when applied to crystal growth, a supersaturation state is generated through the temperature difference based on the temperature dependence of the solubility of the starting material in the ammonia solvent or the like, thereby precipitating a crystal. Concretely, a crystal starting material or a seed crystal is put into a pressure vessel such as an autoclave or the like, sealed up therein and heated with a heater or the like to thereby form a high-temperature region and a low-temperature region in the pressure vessel, while on one hand, the starting material is dissolved and a crystal is grown on the other hand to thereby produce the desired crystal.
The crystal starting material to be used in the ammonothermal method includes a polycrystalline or a single crystal or the like of the same type as that of the nitride crystal to be grown according to the ammonothermal process. Consequently, first, a nitride fine crystal material corresponding to the starting material in the first stage is produced, and then the starting material is grown into a nitride crystal in the second stage (for example, see Patent Reference 1). In this, a massive material could hardly be obtained in the first stage, and therefore, in general, a polycrystalline nitride having smaller particle size, that is, a powdery starting material is used in the second stage. Patent Reference 1 (JP-A 2003-277182) says that it is desirable to use a GaN fine crystal powder having a mean particle size of from 1 to 5 or so as the starting material in the second stage to thereby give a GaN crystal through growth (see paragraphs [0009] to [0010]).
Patent Reference 2 (JP-A 2007-238347) describes efficient production of GaN crystal by the use of multistage crucibles each with a powdery GaN crystal starting material put therein, in which the contact area between the starting material and the solvent is enlarged to thereby increase the dissolution rate of the starting material. In this, it is said that use of boxy crucibles each having one opening port only on the top thereof is preferred. In addition, Patent Reference 2 describes use of a crystal starting material having a particle size of from 10 nm to 10 mm, especially recommending use of a powdery crystal starting material having a particle size of from 50 nm to 1 mm for filling up the gap between the material particles (see [0061]).
Patent Reference 3 (JP-A 2006-83055) and Patent Reference 4 (JP-T 2005-508822) describe a method for producing a polycrystalline nitride to be a starting material. According to Patent Reference 3, there is obtained a needle-like, columnar or prismatic crystal having a primary particle diameter of from 0.1 μm to tens μm and having a maximum length in the long axis direction of from 0.05 μm to 1 mm, and this is said to be used as the starting material for producing a nitride crystal. Patent Reference 4 describes production of a GaN crystal containing an isometric crystal particle having a mean particle size of from about 0.01 to 50 μm, and use of the crystal as the starting material for producing a nitride crystal. In Example 1 in Patent Reference 4, some crystal particles having a diameter of from about 10 to 20 μm are produced along with a large number of crystal particles having a diameter of at most about 1 μm; and in Example 2, there are produced many crystal particles having a diameter of from about 1 to 3 μm and unstriated crystal particles somewhat larger than these.
A nitride crystal such as gallium nitride (GaN) or the like is used for substrates of various semiconductor devices such as light-emitting devices, electronic devices, semiconductor sensors, etc. In order that a nitride crystal can function as a substrate for semiconductor devices, the nitride crystal must have suitable electroconductivity in accordance with the use of various semiconductor devices. In general, the electroconductivity of a nitride crystal is controlled by the carrier concentration and the carrier mobility in the nitride crystal. In order that a nitride crystal can function as a substrate for semiconductor devices, it is important that the nitride crystal has a suitable carrier concentration.
As a method for producing a nitride crystal, there are known a hydride vapor phase epitaxial growth process (HVPE process), an ammonothermal process, etc. The HVPE process comprises introducing a Ga chloride and a Group V element hydride (NH3) into a furnace in a hydrogen current atmosphere, then thermally decomposing them and depositing the crystal generated through the thermal decomposition on a substrate. It is known that, in case where oxygen is used as an n-type dopant in the nitride crystal to be produced according to the HVPE process, the activation ratio is nearly 100% (for example, see Patent Reference 5 (JP-A 2000-44400)).
On the other hand, the ammonothermal process has advantages in that the starting material utilization efficiency there in is better than that in the HVPE process and that the production cost can be reduced.
As so pointed out in Patent Reference 2, the dissolution rate of a powdery nitride crystal starting material is low and the starting material efficiency lowers, and in case where the powdery nitride crystal starting material is used as a material for crystal growth in an ammonothermal process, a large-size crystal could not be obtained efficiently. Patent Reference 2 describes use of a crystal starting material having a particle size of from 10 nm to 10 mm, which, however, says that when the starting material contains large particles of more than 2 mm, it is especially recommended to use a powdery crystal starting material having a particle size of from 10 nm to 2 mm in an amount of at least 10% by mass to thereby fill up the gap between the starting material particles (see [0061])